Uncured rubber surfaces are often marked, or coated, with an identifying marking composition which, after curing the coated rubber in a suitable mold at an elevated temperature and pressure, retains its identifying characteristic.
Conventionally, organic solvent based coating compositions are used for such purpose, primarily because the identifying, or marking, composition is organic solvent-soluble and the organic solvent itself is usually chosen for its ability to wet the uncured rubber surface and, also, to have a suitable drying time.
However, for some applications, it is desired that (1) the identifying marking disappears upon curing the rubber article, and (2) the identifying marking composition is applied as a water based coating.
Water-based coating compositions for application to rubber surfaces, particularly uncured rubber surfaces are sometimes desired, and particularly water-based compositions which can be effectively applied to the uncured rubber surface and which will disappear upon curing the coated rubber.
In another aspect, it would be desired that the applied coating will cause minimal, if any, fouling of the curing mold surface (generally a metal mold surface such as a steel metal surface) against which the coating is pressed under conditions of elevated temperature (120.degree. C.-60.degree. C.) and pressure as the coated rubber is cured. By mold surface fouling it is meant the build-up of substances on the curing mold surface from the rubber product itself, or from a coating on the rubber product, during the curing process. Such mold fouling is well known to those having skill in the rubber curing art.
Thus, various of the preformed, unassembled rubber components may desirably be marked in their uncured state with a water-based coating composition for identification purposes. Such marking may be used for various individual tire components such as, for example, its rubber components of tread, sidewall and carcass.
Indeed, in some instances, it is desired that the uncured rubber components are individually labeled, or identified by a marking composition, or ink, so that they can be individually stored and later identified and utilized in the assembly and fabrication process.
For example, in practice, for the manufacture of rubber tires, a tread rubber might first be formed as an extruded strip of carbon black containing uncured rubber with identifying marking(s) being applied to the rubber. Then, the marked rubber tread strip is built into the tire and the resulting tire assembly cured at an elevated temperature.
It is important that the aqueous based marking composition desirably have the properties of chemical and mechanical stability, visibility (particularly on black-colored rubber surfaces), co-curability with the rubber substrate to enhance adhesion to the rubber surface, ease of application, wettability to the uncured rubber substrate surface and cause minimal mold fouling during general use.
Therefore, for this invention it is desired that an identifying coating composition be applied to an uncured rubber substrate which, upon curing the coated rubber substrate, disappears or at least substantially disappears so that it is not easily recognizable without a very close detailed inspection.
Although not necessarily limited thereto but as a preferred aspect, it is desired the uncured rubber substrate contains carbon black, as well as the curative, and is, therefore, black in color. In such instance, it is desired that the coating composition has a color that visibly contrasts with black, such as a white color. Thus, zinc stearate is prescribed.
For the coating itself, it is also desired that it is film forming on the rubber substrate surface in that a continuous film on the cured rubber surface is obtained with a minimal, if any, cracking of its surface for a reasonable period of time after its application, is compatible with conventional rubber compositions, and, in its aqueous application stage, has a suitable drying time on the rubber surface at conventional ambient temperatures of about 20.degree. C. to about 30.degree. C.
Zinc stearate has a distinguishing white color against a background of a black colored rubber surface. However, a zinc stearate by itself is unsatisfactory for coating a rubber substrate surface because it does not have flexibility and is not sufficiently film forming.
Zinc stearate, when mixed with a rubber solution in an organic solvent, can be applied to a black colored rubber surface and used as a distinguishing white coating. However, as hereinbefore pointed out, it is desired that an aqueous mixture is used. Indeed, in one aspect of the invention it is desired that the coating composition be applied to the rubber substrate as a water based dispersion and/or emulsion.
Indeed, it is desired that the coating does contain a colorant, such as zinc stearate, of a color which contrasts with the rubber, particularly black colored rubber. It is further desired that the coating have a sulfur curable polymeric binder for the zinc stearate in order to form a suitable continuous film on the rubber surface, have a degree of flexibility and be co-curable with the rubber substrate itself.
Thus, it is envisioned that the marking composition would, basically, be composed of the aforesaid zinc stearate filler and a curable polymer binder, particularly a compatible, sulfur curable rubber binder, for the filler. It is to be appreciated that it is contemplated that additional materials may be added to the coating composition such as, for example, wetting agent, emulsifier(s) and flattening agent(s).
However, for an aqueous mixture, an inherent difficulty presents itself in using zinc stearate with a rubber latex.
Indeed, conventional rubber latices made with natural soaps (fatty acids and rosin acid soaps) are prone to destabilize in the presence of multivalent salts such as zinc stearate.
In the description of this invention, the terms compounded rubber, rubber compound and rubber composition as well as rubber substrate may be used somewhat interchangeably to refer to rubber which has been mixed with compounding ingredients, typically including carbon black, zinc oxide, zinc stearate or stearic acid, pigments resin(s) and curatives, or curing agents.